Russia’s President Dmitry Medvedev has addressed the Polish nation and vowed that a thorough investigation into the cause of the catastrophe will be carried out, in full co-operation with the Polish side. April 12 has been declared a day of mourning in Russia.

According to the Russian Emergencies Ministry, 96 people – including the president, First Lady Maria Kaczynska and dozens of Poland’s most senior politicians – died in the tragic accident on Saturday morning. The delegation was on its way to the Russian city of Smolensk to attend a ceremony marking the 70th anniversary of the Katyn Massacre, which took place in a forest west of the city. Over 20,000 Polish officers were executed in 1940 by order of Joseph Stalin.

President Medvedev sent his condolences to the acting President of Poland, Marshal of the Sejm – parliamentary speaker – Bronislaw Komorowski on the Saturday’s tragedy.

“I, as with all Russian citizens, have met the news of the tragedy with deep and sincere shock - the death in a plane crash of the Republic of Poland's President Lech Kaczynski, his wife, and the members of the large Polish delegation,” his letter reads, as quoted on the Kremlin website. He said Russia shares with Poland their grief and mourning.

A commission headed by Prime Minister Vladimir Putin has been set up to investigate the crash of the Polish presidential TU-154 plane.

The Russian premier has sent condolences to his Polish counterpart, Donald Tusk.

"Vladimir Putin called Prime Minister Donald Tusk and expressed his condolences to him personally and the entire Polish nation in regard to the tragic airplane crash outside Smolensk," Putin’s press secretary Dmitry Peskov said as quoted by RIA Novosti.

Lech Kaczynski (AFP Photo / Attila Kisbenedek

Tusk was reportedly in tears when he was told about the accident.

Foreign Minister Sergey Lavrov – who is currently on an official visit to Kiev – has promised that the Russian side will do everything possible so that all issues related to the accident can be resolved swiftly.

The head of Russia’s State Duma Foreign Affairs Committee, Konstantin Kosachev, Interfax agency writes, referred to events as to a “huge human” and “political tragedy”. He said he was in a state of shock over the news. “Katyn has claimed new victims,” Kosachev added.

US President Barack Obama has called Polish Prime Minister Tusk to express his “deepest condolences”.

"Our thoughts and prayers are with the Kaczynski family, the loved ones of those killed in this tragic plane crash, and the Polish nation,” he said in a statement published on the White House website. “Today's loss is devastating to Poland, to the United States, and to the world. President Kaczynski was a distinguished statesman who played a key role in the Solidarity movement, and he was widely admired in the United States as a leader dedicated to advancing freedom and human dignity."

German Chancellor Angela Merkel, Deutsche Welle reports, has sent a letter to Tusk. "All of Germany stands by Poland with empathy and solidarity in this difficult hour," she wrote.

French President Nicolas Sarkozy has also expressed his sorrow over the fatal accident.

"A patriot, Lech Kaczynski devoted his life to his country. A tireless defender of the ideas in which he believed, he always fought with conviction for the values that founded his coming into politics: these are democracy, liberty and the fight against totalitarianism," his statement reads.

With the death of Polish president “France has lost a friend, sincerely devoted to the development of relations between the two states.”

Latvia has also paid its tribute to Kaczynski. President Valdis Zatlers said the Polish leader and his wife were “true friends to Latvia.”

The Lithuanian government, sharing the grief of Polish people, is planning to announce mourning over the Smolensk plane crash.

Condolences are pouring in to Warsaw from all over the world, including from the OSCE Chairperson, Kazakhstan's Foreign Minister Kanat Saudabayev, Ukraine’s President Viktor Yushchenko and premier Nikolay Azarov.

British Prime Minister Gordon Brown said he believes the “whole world will be saddened and in sorrow” as a result of the tragic events.

“On behalf of all of NATO, and myself personally, I express my most profound condolences to the people of Poland, and to the families of President and Mrs. Kaczynski and all those who have died in this terrible accident. This is a tragedy for them and for Poland. My thoughts are with them today.”

The United Nations’ Secretary General Ban Ki-moon expressed his deep condolences to Poland and said he was "shocked" to learn about the death of Kaczynski and other officials.

Seven days of mourning have been announced in Poland.

"I have just signed a decision on the announcement of a national week of mourning," acting president Komorowski said, as quoted by RIA Novosti. "We must all come together in the face of this massive national tragedy," he added. "Today, there is no left wing and no right wing – there is no difference. We all feel for the families of the dead."